A typical field effect transistor (FET) comprises a number of layers and they can be configured in various ways. For example, an FET may comprise a substrate, a dielectric, a semiconductor, source and drain electrodes connected to the semiconductor and a gate electrode. When voltage is applied between the gate and source electrodes, charge carriers are accumulated in the semiconductor layer at its interface with the dielectric resulting in the formation of a conductive channel between the source and the drain and current flows between the source and the drain electrode upon application of potential to the drain electrode.
FET's are widely used as a switching element in electronics, for example, in active-matrix liquid-crystal displays, smart cards, and a variety of other electronic devices and components thereof. The thin film transistor (TFT) is an example of a field effect transistor (FET). The best-known example of an FET is the MOSFET (Metal-Oxide-Semiconductor-FET), today's conventional switching element for high-speed applications. Presently, most thin film devices are made using amorphous silicon as the semiconductor. Amorphous silicon is a less expensive alternative to crystalline silicon. This fact is especially important for reducing the cost of transistors in large-area applications. Application of amorphous silicon is limited to low speed devices, however, since its maximum mobility (0.5-1.0 cm2/V·sec) is about a thousand times smaller than that of crystalline silicon.
Although amorphous silicon is less expensive than highly crystalline silicon for use in TFT's, amorphous silicon still has its drawbacks. The deposition of amorphous silicon, during the manufacture of transistors, requires relatively costly processes, such as plasma enhanced chemical vapor deposition and high temperatures (about 360° C.) to achieve the electrical characteristics sufficient for display applications. Such high processing temperatures disallow the use of substrates, for deposition, made of certain plastics that might otherwise be desirable for use in applications such as flexible displays.
In the past two decades, organic materials have received significant attention as a potential alternative to inorganic materials, such as amorphous silicon, for use in semiconductor channels of FET's. Compared to inorganic materials, that require a high-temperature vacuum process, organic semiconductor materials are simpler to process, especially those that are soluble in organic solvents and, therefore, capable of being applied to large areas by far less expensive processes, such as roll-to-roll coating, spin coating, dip coating and microcontact printing. Furthermore organic materials may be deposited at lower temperatures, opening up a wider range of substrate materials, including plastics, for flexible electronic devices. Accordingly, thin film transistors made of organic materials can be viewed as a potential key technology for plastic circuitry in display drivers, portable computers, pagers, memory elements in transaction cards, and identification tags, where ease of fabrication, mechanical flexibility, and/or moderate operating temperatures are important considerations. However, to realize these goals, OFET semiconductor and dielectric components should ideally be easily manufactured using high-throughput, atmospheric pressure, solution-processing methods such as spin-coating, casting, or printing.
To date in the development of organic field effect transistors (OFET's) considerable efforts have been made to discover new organic semiconductor materials and optimizing properties of such materials. The overall performance of an OFET is dependent on a number of factors such as the degree of crystallization and order of organic semiconductor layer, charge characteristics and trap density at the interfaces between dielectric and organic semiconductor layers, carrier injection ability of the interfaces between source/drain electrodes and organic semiconductor layers. Although, the gate dielectric layer is intended to ensure a sufficiently good electrical insulation between the semiconductor and the gate electrode it plays an important role in the overall performance of an OFET. In particular, the gate dielectric permits the creation of the gate field and the establishment of the two-dimensional channel charge sheet. Upon application of a source-drain bias, the accumulated charges move very close to the dielectric-semiconductor interface from the source electrode to the drain electrode. Since the charge flow in organic semiconductor occurs very close (˜1 nm) to the dielectric interface, it is important to optimize chemical and electrical behavior of the dielectric layer.
Besides these factors, the dielectric surface morphology has a great effect on carrier or charge mobility of the semiconductor. The surface morphology of the dielectric material and variations in its surface energies [for example, surface treatment via self-assembled monolayers (SAM's)] have been shown to modify the growth, morphology, and microstructure of the vapor/solution-deposited semiconductor, each of these being a factor affecting mobility and current on/off ratio, the latter being the drain-source current ratio between the “on” and “off” states, another important device parameter. The properties of the dielectric material can also affect the density of state distribution for both amorphous and single-crystal semiconductors.
Gate dielectric materials for OFET's can be divided into inorganic and organic materials. Inorganic dielectric materials like silicon oxide (SiO2), silcon nitride (SiNx), aluminum oxide (AlOx), and tantalum oxide (TaOx) are conventionally deposited via chemical vapor deposition (CVD) and plasma enhanced CVD methods which are high temperatures (>300° C.) processes and not compatible with polymeric substrates. Lower processing temperatures usually lead to poor quality films with pinholes, resulting in poor insulating properties. As a result it is necessary to use thick layers (more than 100 nm) to ensure sufficiently good insulator properties which results in increased supply voltages for operation of such circuits. Another widely used process is ion beam deposition, but it needs high vacuum and expensive equipment that are incompatible with the goal of very low cost production. Similarly, use of other high dielectric constant inorganic materials as barium zirconate titanate (BZT, BaSrTiO3) and barium strontium titanate (BaSrTiO3, BST) need either a high firing temperature (400° C.) for the sol-gel process, or radiofrequency magnetron sputtering, which also requires vacuum equipment, and can also have stoichiometric problems.
In addition to higher temperature processing, inorganic insulating layers generally require interfacial modification before they can be used with an organic semiconductor. It has been shown that the presence of polar functionalities (like —OH groups on SiO2 surface) at the dielectric-organic semiconductor interface trap charges which results in lowers carrier mobility in organic semiconductors.
Most organic materials used in OFET's cannot withstand the high processing temperatures used with conventional inorganic materials. For example, the 200° C. or higher temperatures needed to process conventional inorganic materials would at the very least cause a polymeric substrate to deform, and might cause further breakdown of the polymer or even ignition at high enough temperatures. Deformation is highly undesirable, since each layer of the structure has to be carefully registered with the layers below it, which becomes difficult to impossible when the layers below it are deformed due to processing temperatures.
As an alternative to inorganic gate dielectrics, insulating polymers have been proposed for fabrication of OFET's, including poly(4-vinyl phenol) (PVP), poly(methyl methacrylate) (PMMA), poly(styrene) (PS), poly(perfluorobutenylvinylether) (CYTOP), and poly(acrylic acid) (PAA). U.S. Pat. No. 7,298,023 (Guillet et al.) discloses use of organic insulator (or dielectric) comprising a base polymer of PVDC-PAN-PMMA copolymer with the general formula (—CH2Cl2—)x—(—CH2CH(CN)—)y—(CH2C(CH3)(CO2CH3))z, wherein x, y, z, in each case, independently from one another, can assume values between 0 and 1 for use in OFET's and organic capacitors. However, the presence of polar groups at the dielectric interface can create dipolar disorder which lowers the carrier mobility. U.S. Patent Application 2008/0283829 (Kim et al.) relates to an organic insulator composition including a crosslinking agent and a hydroxyl group-containing oligomer or hydroxyl group-containing polymer is provided. However, the presence of hydroxyl groups at the organic semiconductor-gate dielectric interface is not desirable as hydroxyl groups trap charges.
U.S. Pat. No. 6,232,157 (Dodabalapur et al.) relates to the use of polyimide as material for organic insulating films, but such insulating films require a high temperature heat treatment to obtain chemical resistance.
U.S. Pat. No. 6,617,609 (Kelley et al.) relates to coating a thin siloxane polymeric layer on a gate dielectric (that is usually SiO2). Device performance was shown to improve when a siloxane polymeric layer was present but this approach is limited in scope since it requires inorganic oxide gate dielectrics (claim 11). Moreover, the siloxane polymer is not on top of an organic polymer gate dielectric. Similarly, U.S. Pat. No. 7,352,000 (Kelley et al.) relates to an OFET comprising a thin substantially nonfluorinated polymeric layer interposed between a gate dielectric and an organic semiconductor layer. Although this publication mentions polymeric gate dielectric materials such as poly(vinylidene fluoride) (PVDF), cyanocellulose, polyimides, and epoxies, it specifically teaches the use of inorganic materials for the gate dielectric, and requires the coating of multiple layers, which is difficult and costly.
U.S. Patent Application Publication 2009/0166613 (Lee et al.) describes a composition for forming a gate insulating layer of an OFET comprising a polyacrylate dielectric and pentacene as semiconductor with mobilities in the range of 0.19-0.25 cm2/V·s, which are significantly lower than those reported by Park et al., Thin Solid Films 515, 4041-4044 (2007).
U.S. Pat. No. 7,279,777 (Bai et al.) describes cyano-functional polymers for Al2O3 dielectric surface modification and OFET devices.
Since these polymer dielectrics are not crosslinked, the resulting dielectric film cannot be subjected to any processes involving the use of solvents. To address this problem various cross linked polymer dielectrics have been developed for example as described by Halik et al. [Journal of Applied Physics 93, 2977 (2003)]. In addition, U.S. Pat. No. 7,482,625 (Kim et al.) describes blending polyvinyl phenol with another polymer for certain physical, chemical and electrical characteristics. However, this approach has limited application since a high temperature of about 200° C. is required to attain crosslinking.
Low operating voltage is essential for various OFET's applications, such as portable displays, smart cards, and radio frequency identification tags. Furthermore, patchable electronics such as smart patches and smart textiles must be operated at low voltages because they are worn on the human body. To operate at low voltage, OFET's must have a dielectric layer with high dielectric constant. For example, ferroelectric insulators, such as BaSrTiO3(BST)), Ta2O5, Y2O3, and TiO2 and inorganic insulators, such as PbZrxTi1-xO3 (PZT), Bi4Ti3O12, BaMgF4, SrBi2(Ta1-xNbx)2O9, Ba(Zr1-xTix)O3 (BZT), BaTiO3, SrTiO3, and Bi4Ti3O12 have been used as materials for inorganic insulating films. However, these inorganic oxide materials do not have any significant advantages over conventional silicon materials in terms of processing.
Polymer materials are usually not appropriate for use in low voltage applications due to their low dielectric constant (k). There have been many attempts to obtain a high capacitance with gate dielectrics by reducing their thickness or using polymer-inorganic composites to increase the dielectric constant (k) to produce low voltage operating OFET's
For example, Japanese Patent 3,515,507 (Aoki et al.) discloses an organic polymer and an inorganic material that are mixed to provide insulating film with flexibility and high dielectric constant. In accordance with this reference, a powder obtained by mechanically grinding a ferroelectric material such as barium titanate is dispersed in an organic polymer to compensate the dielectric constant of the resulting insulated gate film and hence lower the gate voltage required for the operation of OFET. However, when this method is used, the thickness of the insulating film is limited to the size of the inorganic material thus ground. Furthermore, since a solid material is dispersed in an organic polymer solution, an uneven dispersion is formed, possibly causing the generation of local electric field and concurrent dielectric breakdown during the operation of transistor. Importantly, since the inorganic material is merely present in the organic polymer and thus does not compensate the chemical resistance of the insulating film, the resulting insulating film cannot be subjected to any processes involving the use of solvents.
Japanese Patent Publication 2003-338551 (Shindo) discloses a technique of forming a thin ceramic film as an insulating film on the surface of silicon wafer by a sol-gel method allowing a low temperature treatment. The resulting thin ceramic film can be prevented from being cracked, making it possible to efficiently produce electronic parts having a high reliability. However, the thin ceramic film is an insulating film made of an inorganic material that can be applied to silicon wafer, which is nonflexible and hard, but it cannot be applied to flexible substrates.
U.S. Patent Application 2010-0230662 (Chen et al.) discloses gate insulating layer comprising an azole-metal complex compound. However, pentacene based OFET having an azole-metal complex compound as a dielectric shows lower mobility and poor current on/off ratio.
U.S. Patent Application 2010-0051917 (Kippelen et al.) discloses embodiments of OFET's having a gate insulator layer comprising organic polymer, specifically poly(vinyl phenol) (PVP), nanocomposites incorporating metal oxide nanoparticles (for example, barium titanate (BaTiO3), strontium titanate (SrTiO3), and barium zirconium titanate (BaZrTiO3)) coated by organic ligands and methods of fabricating such OFET's.
Usually dispersions of metal oxide nanoparticles in a polymer matrix are not homogeneous. In an attempt to obtain a uniformly dispersed organic-inorganic mixture system as an ordinary material technique, it has been practiced to prepare a composite film from a mixture of a solution of metal alkoxide that is a precursor of inorganic oxide and an organic polymer solution by a sol-gel method. In this case, it is expected that as the dispersion of organic polymer is made more on monomolecular level, the thermal stability of the organic polymer is more enhanced. Thus, when a polymer or molecules capable of making hydrogen bond such as hydroxyl group or electrostatic mutual action are present in a metal alkoxide solution, a sol-gel polycondensation is formed selectively on the surface of the compound to form a dried gel.
A composite film of poly(methyl methacrylate-co-methacrylic acid) (PMMA-co-MAA)/sol-gel-derived TiO2 has been used as gate dielectric layer in pentacene based OFET's. However, surface roughness of such films is quite high (about 2.1 nm). In the case of deposition of small semiconductor molecules, such as pentacene, molecular orientation and grain morphology depend strongly on the surface roughness and energy of the underlying film. Surface smoothness of poly(4-vinylphenol)-composite is better (about 1.3 nm) and pentacene OFETs show smaller threshold voltage (Kim et al. J Am. Chem. Soc. 132, 14721, 2010).
Korean Patent Publication 2007-0084643 (Kim et al.) discloses a gate dielectric composition wherein the gate insulating layer may be formed of typical organic and/or inorganic compounds. Examples of such organic compounds include poly(vinyl phenol), poly(methyl methacrylate), polyacrylate and poly(vinyl alcohol), and examples of inorganic compounds include SiNx (0<x<4), SiO2 or Al2O3. A poly(vinyl phenol) copolymer mixed with a crosslinking agent and an organic-inorganic hybrid insulator can be used. On top of the gate insulating layer a thin film of a fluorine-based polymer can be applied. This publication teaches the use of poly(methyl methacrylate) and a polyacrylate as a dielectric material but it requires coating a thin film of fluorine containing polymer on top of it.
U.S. Patent Application Publication 2009/0224234 (Kim et al.) discloses a photocurable inorganic/polymer composite including metal oxide nanoparticles by mixing a metal oxide precursor and a photocurable transparent polymer, and converting a part of the organic film into nano-inorganic particles through sol-gel and photocuring reactions. However, a strong protic acid (nitric acid) or a base like sodium hydroxide is required as catalyst to carry out sol-gel reaction in thin film. This method introduces mobile ionic impurities in dielectric layer that could be problematic in electrical applications.
Although various polymer dielectric compositions are known, a number of problems still remain in terms of the process of making such dielectric layers and improving overall performance in OFET's. As discussed before, to increase the dielectric constant of polymer dielectric materials, a number of polymer-inorganic composites, by adding metal oxide nanoparticles and making metal oxide nanoparticles by sol-gel method, are known. However, these methods do not result in crosslinking of the dielectric layer, which is a key requirement in solution processable OFET's. To achieve crosslinking photocurable resins have to be used.
Thus, there is a need to increase the dielectric constant of a polymeric dielectric layer and to provide crosslinking so that solution processing is possible. With the difficulty in balancing all desired properties in mind, there continues to be research to find such useful polymeric dielectric materials.